Mass range and atomic mass units (amu)

The mass range refers to the range of mass-to-charge ratios (m/z) that a mass spectrometer can effectively measure. Both the Xinceyuan ProC series and the ProC-1C/ProC-1S systems are specified as having a mass range of 1–500 amu (atomic mass units), meaning that ions ranging from hydrogen (m/z 2) to m/z 500 can be covered in a single scan; MSI LDPI is specified to detect molecular weights of approximately 70–1000, whilst MSI DPI covers the range of lipids and secondary metabolites commonly found in biological tissues.

amu (atomic mass unit, now also denoted as u or Da) is a unit of measurement for atomic and molecular masses, defined as approximately 1/12 of the mass of a carbon-12 atom. In mass spectrometry, the m/z value of a monocharged ion is almost equal to its molecular mass (expressed in amu). Consequently, a ‘mass range of 1–500 amu’ roughly corresponds to substances with molecular weights of 1–500 daltons—which covers the vast majority of small molecules in environmental gases, flue gas components, and many secondary metabolites and lipid precursors found in plant and animal tissues.

Understanding the mass range helps in determining suitable applications. When studying hydrogen, oxygen and nitrogen oxides in the air (all < 50 amu), or nicotine (m/z 162) and complex lipids (typically 500–1000 amu) in flue gas, it is necessary to confirm that the instrument’s range covers the targets. New Source has set the mass range of its online mass spectrometers at 1–500 amu; combined with high resolution and a wide dynamic range, this enables a single instrument to detect both the smallest gas molecules and medium-sized organic molecules, making it suitable for applications requiring ‘broad coverage’ such as catalysis, environmental analysis, flue gas analysis and food analysis; whilst the imaging mode, utilising LDPI, extends the upper limit to approximately 1000 amu, which is sufficient to cover most lipids and metabolites.

It is important to clarify a common misconception: mass range is not the same as mass resolution. The range answers the question ‘how large or small an ion can be measured’, whilst resolution answers ‘whether two adjacent m/z values can be separated’. An instrument may have a very wide range (e.g. 1–500 amu) but limited resolution, or it may have a narrower range yet extremely high resolution. By specifying the range whilst also stating a resolution of ‘better than 3,500 at m/z = 92’, New Ceyuan clearly distinguishes between these two metrics to avoid confusion amongst users. For real samples, both are indispensable: insufficient range will result in ‘missing large molecules’, whilst insufficient resolution will result in ‘failure to distinguish between adjacent peaks’.